This invention relates to heat-treating methods and particularly to application of eco-friendly starch solution as quenchant for quenching heated metal parts fabricated from steel, alloy steel, aluminum and other nonferrous alloys.
The present invention claims priority to Indian utility patent application (Title: Method of Quenching) No. 94/MUM/2001, filed in India on Jun. 7, 2001 which claims priority to Indian provisional patent application No. 94/MUM/2001, filed Jan. 25, 2001 in India.
Hardening of steel components is one of the most commonly practiced heat treatment operations in the steel industry. The hardening process comprises heating the steel components to austenitizing temperature (xcx9c800-1000xc2x0 C.), soaking at that temperature for thermal homogenization, followed by quenching in an appropriate medium to room temperature. Quenching is a process whereby a steel component heated to a given elevated temperature is rapidly cooled by immersion in a quench bath containing compositions having a high heat-extracting potential such as air, water, brines, oils or polymer solutions.
The rate of cooling is an important parameter during the quenching process. Cooling rates are dependent on factors such as the size, shape and composition of the component being quenched as well as the composition, concentration, degree of circulation and temperature of the quench bath.
In steel, depending on the cooling rate during quenching, the austenite phase can be transformed into a variety of other phases such as ferrite, pearlite, bainite and martensite. Of the various phases, ferrite is the softest phase, whereas martensite is the hardest phase. Slow cooling rate results in formation of ferrite phase, whereas fast cooling rate provides martensite phase. Formation of martensite phase is generally the main aim of hardening treatment.
It must be noted that cooling rates are not uniform throughout a component; surface regions are better able to dissipate heat and thus cool faster than interior regions. The difference in cooling rates and temperature gradient within the component produce thermal stresses. If the cooling rate is very high, the thermal stresses can result into warping, distortion and even cracking of the component.
Therefore, during the quenching process, the cooling rate of a component should be fast enough to prevent formation of soft ferrite phase, but not too fast to prevent distortion and cracking. The quantification of too fast or too slow cooling rate mainly depends on the steel grade, and accordingly appropriate quenching media is selected for a particular grade of steel. For example, medium alloyed air hardening grade steels (AISI A2, A3, A4, A10, etc.) can be hardened by air cooling, oil hardening grades (AISI O1, O2, O6, etc.) can be hardened through quenching in oil, whereas water hardening grades (AISI W1, W2, W3, etc.) require high cooling rates which can be obtained only through water and brine quenching.
Of the various quenching media, oil is one of the most common quenching media in the industry. Generally, quenching oil provides moderate cooling rate and therefore results in minimal distortion in the component. Therefore, many of the precision components such as gears and bearing rings are hardened by oil quenching.
Although quenching oil exhibits this highly advantageous cooling response, currently, the use of oil as a common quenching medium for hardening of steel is associated with several environmental liabilities such as oil fumes, smoke emissions, fire hazard, oil spills, leaking underground storage tanks, ground water contamination and waste oil disposal liabilities. With increasing environmental awareness as well as strict regulations, the use of oil as quenching media is being discouraged in many of the environmentally conscious countries. For example, in USA, used quenching oil is considered a hazardous waste and its disposal is regulated by EPA""s strict used oil management standards. In the comprehensive technology roadmap xe2x80x9cVision 2020xe2x80x9d prepared by ASM Heat Treating Society, in close collaboration with leaders in heat treatment industries, replacement of oil as a quenching media has been given very high importance to achieve zero emissions from heat treatment industry.
Significant efforts have been put in towards the development of synthetic organic polymer quenching media as an alternative to Oil. However, most of these synthetic quenchants are proprietary and expensive which makes their usage very limited.
U.S. Pat. No. 3,022,205 discloses an aqueous quenchant medium containing between 0.2 g and 4.5 g. per gallon of water, of an ethylene oxide polymer having a molecular weight of between 100,000 and several million.
U.S. Pat. No. 3,220,893 discloses a metal quenchant medium containing an aqueous solution of an oxyalkylene polymer containing both oxyethylene units and higher molecular weight oxyalkylene units such as units derived from propylene oxide. The polymers are further described as having an oxyethylene to oxyalkylene ratio by weight of from about 70:30 to about 90:10, and an average molecular weight of from 600 to 40,000. The specific polymer exemplified is a polyglycol containing 75 percent by weight of oxyethylene units and 25 percent by weight of oxypropylene units, having a viscosity of about 90,000 Saybolt seconds at 100 degrees F. and an average molecular weight of from about 12,000 to about 14,000.
U.S. Pat. No. 3,475,232 discloses an aqueous quenchant containing a normally liquid water soluble oxyalkylene polymer having oxyethylene and higher molecular weight oxyalkylene units, and a water soluble alcohol selected from the group consisting of glycerol, glycols containing from 2 to 7 carbon atoms, and mono-lower alkyl ethers of said glycols in which the alkyl group contains from 1 to 4 carbon atoms. A polymer comprising about 75 percent by weight of oxyethylene units and about 25 percent by weight of oxypropylene units, having a viscosity of about 150,000 Saybolt seconds at 100 degrees F. is particularly preferred.
U.S. Pat. No. 4,381,205 discloses a metal quenching process using an aqueous quenchant bath containing from about 0.5 to about 50% by weight of the bath, of a liquid, water-soluble or water dispersible capped polyether polyol.
An object of this invention is to provide a suitable eco-friendly natural quenchant, which can replace quenching oil from the heat treatment industry. Commercial aspects such as being low-cost and easily available have also been considered.
It is a further object of this invention that the quenchant should have following characteristics:
(i) The quenching intensity should be at least equivalent to that of oil, and at the same time it should not be very high to introduce distortion and cracking in the as quenched component.
(ii) It should be reusable and possess long shelf life. It should not age appreciably with time and repeated use.
(iii) It should not generate fumes during quenching operation.
(iv) The spurting should be minimal during the quenching operation. This is especially important for salt bath hardening, where vigorous spurting occurs during water quenching.
(v) It should be biodegradable and the disposal of used quenchant should not pose any environmental problem.
(vi) In addition to the above characteristics, commercial issues such as: low cost and availability must be addressed.
In a preferred embodiment of the present invention a method of heat treating a heated metal part is provided, comprising the steps of (a) preparing a quenching bath comprising starch dissolved in water; and (b) immersing the heated metal part in the quenching bath for a period of time to accomplish the heat treating. The starch in one embodiment may be cornstarch, and may vary in concentration from two percent to four percent inclusive by weight. The starch in another embodiment is laundry starch, and in some embodiments may vary in concentration from two percent to three percent by weight. In other embodiments the starch is derived from one of potato, rice or tapioca.
In various embodiments the water is preferably at a temperature of from eighty-five to ninety-five degrees Celsius inclusive. In a preferred method, in step (a), after adding the starch to the water, the slurry so formed is left unmolested for from five to fifteen minutes inclusive, prior to step (b). Further, the temperature of the resulting solution is allowed to decline to room temperature before step (b). In some embodiments there is a further step for adding formaldehyde, as a preservative, to the solution prior to step (b). The concentration of formaldehyde is typically from about one-half to one percent by weight.
In another aspect of the invention a quenching liquid for heat treating a heated metal part is provided, comprising a volume of water as a vehicle, and starch dissolved in the water vehicle. In a preferred embodiment the starch is cornstarch. The concentration of cornstarch in the water vehicle is preferably from two percent to five percent inclusive by weight. In some other embodiments the starch is laundry starch, and concentration of laundry starch in the water vehicle is from two percent to three percent by weight. In other embodiments the starch is derived from one of potato, rice or tapioca, and other sources may be used as well.
In preparing the quenchant the water vehicle is preferably at a temperature of from eighty-five to ninety-five degrees Celsius inclusive while the starch is dissolved in the water vehicle, and, after adding the starch to the water vehicle, the slurry so formed is left unmolested for from five to fifteen minutes inclusive. After dissolving the starch in the water, the temperature of the resulting solution is allowed to decline to room temperature before use as a quenchant ion heat treating.
In many embodiments a formaldehyde component is added as a preservative, and the concentration of formaldehyde is preferably about one percent by weight.
In yet another aspect of the invention a method for preparing a quenchant for heat treating is provided, comprising the steps of (a) determining, for a particular part to be treated, a material grade, critical temperature, and a desired cooling rate for the part to be treated; and (b) tailoring the quenchant to the task by dissolving an available starch material in a solvent to match the characteristics from step (a), including the desired cooling rate.
In various embodiments of the invention taught in enabling detail below, for the first time, a truly environmentally safe quenchant for heat treating is provided, which is also economical and effective.